in the molecule of the
substance in question of one or more _asymmetric carbon atoms_ and
manifests itself in differences in the optical activity of the compound.[1]
Thus, in the formula for glucose shown above there appear four asymmetric
carbon atoms, namely, those of the four secondary alcohol groups (in the
terminal, or primary alcohol, group, carbon is united to hydrogen by two
bonds, and in the aldehyde group it is united to oxygen by two bonds).
Similarly, fructose contains three asymmetric carbon atoms.

As an example of how the presence of these asymmetric carbon atoms results
in the possibility of many different space relationships, the following
graphic illustrations of the supposed differences between dextro-glucose
and levo-glucose, and between dextro- and levo-galactose, may be cited.[2]

_d_-glucose _l_-glucose _d_-galactose _l_-galactose

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
| | | |
H-C-OH H-C-OH H-C-OH HO-C-H
| | | |
H-C-OH H-C-OH HO-C-H H-C-OH
| | | |
HO-C-H H-C-OH HO-C-H H-C--OH
| | | |
H-C-OH HO-C-H H-C-OH HO-C-H
| | | |
CHO CHO CHO CHO

Comparisons of the above formulas will show that the difference
between the formulas for _d_- and _l_-glucose lies in the arrangement of
the H atoms and the OH groups around the two asymmetric carbon atoms next
the aldehyde end of the chain; while the _d_- and _l_-galactoses differ in
that this arrangement is in the reverse order around all four of the
asymmetric carbons. By similar variations in the grouping around the four
asymmetric atoms, it is possible to produce the sixteen different space
arrangements shown on page 37 for the groups of an aldohexose. Sugars
corresponding to fourteen of these different forms have been discovered,
three of which are of common occurrence in plants, either as single
mono-saccharides or as constituent groups in the more complex
carbohydrates; the remaining two forms have only theoretical interest.

Si